You use Firefox and/or Thunderbird, and you get a list of dictionaries with plenty of useless entries: here's how to remove them.

﻿When you use these software, the dictionaries installed might include regional variants. I use English, French, and Spanish languages: each of these include several regional dictionaries ... that I don't use. So I am stuck with a long list that's no use ... apart from getting lost in it, when I am looking for a dictionary I want to use.

To short down the list, you only need to remove dictionaries:

go to the directory /usr/lib/firefox/dictionaries

delete all the dictionary files that you are not interested in: you will need root access(using sudo from the terminal or by opening a file explorer - Nautilus, PCManFM ...- in root mode).

restart Firefox and Thunderbird, the dictionaries list has shorten.

WATCH OUT: it is likely an update will reinstall the dictionaries and that you will have to start over the same operation. So, it might be handy to write a little script (shell) to perform the operations stated above.